Ammonium nitrate which is used for example in the production of fertilizers is desirably prepared in the form of granules having a grain size from about 1 to about 4 millimeter average diameter. All granulation processes, however, produce granules outside of this range, requiring a screening stage to eliminate the fines and oversize material. Typically, the oversize material is crushed, and fed together with the fines back to the granulation process. Therefore, the efficiencies of granulation processes depend upon the amount of fines and oversize material which must be returned to the process. A greater amount of this outsize material will require additional equipment and energy for processing.
Dual stage granulation processes are generally known in the art, wherein two different granulation devices are combined in series to initially granulate a material and thereafter to reprocess and granulate the outside material produced in the initial process. It is generally known, for example, to combine prilling and drum granulation, or drum granulation and fluidized-bed granulation, in series. German Patent No. DE-2,164,731 and French Patent No. FR-7,246,363, for example, describe the grain enlargement of ammonium nitrate fertilizers by diverting a portion of the feed stream for a prill tower to a second stage granulating device in which the diverted melt stream is sprayed onto the prills in a drum granulator. This process requires the initial production of prills, which are thereafter enlarged by the addition of further melt material in the drum granulator, to form the end product. The extent to which the formation of outside material can be suppressed is limited, and is dependent on the characteristics of the ammonium nitrate liquid phase, the granulation temperatures, and the use of additives.